SpyOn jasmine doesn't work if function is assigned to a variable

I have a spyOn on object's field:

const object = { function?: Function }

spyOn(object, 'function').and.returnValue(Promise.resolve(result));

method I want to test uses this function

If it is used this way:

object.function!();

then it works properly

but if it is used like this:

const func = object.function;
if(!func)
 return;
func();

then spy isn't called How to properly make a setup of spy in the second approach?